For months,
Hillary Clinton
has all but ignored her chief rival for the Democratic presidential nomination, Sen.
Bernie Sanders.
That’s about to change.

With Democratic candidates set to debate for the first time on Tuesday, Mrs. Clinton has the tricky task of directly confronting the upstart Vermont senator, who has gained in polls, without alienating his passionate supporters. For Mr. Sanders, the debate is an opportunity to widen his appeal and persuade skeptical Democrats that he would be a viable nominee.

Since midsummer, Mrs. Clinton has been laying out policy proposals on energy, health care, gun control, Wall Street regulation and more. But they have been overshadowed by persistent questions about her private email arrangement as secretary of state and by the rise of Mr. Sanders, who is drawing crowds with an unapologetic call for a political revolution that will confront the “billionaire class.”

Now, Mrs. Clinton looks to highlight her policy agenda, reassure Democrats and get them excited about her again as their standard-bearer, Clinton advisers and other Democrats say. The nationally televised forum will also be watched by independent voters, with whom Mrs. Clinton’s standing has fallen.

“When she came into this race she had near-universal favorable ratings with Democrats,” said Mo Elleithee, executive director of the Georgetown Institute of Politics and Public Service. “She needs to come in and remind people what they like about her, what they still like about her, and make an assertive, compelling case for herself that is about more than just inevitability.”

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Tuesday’s debate in Las Vegas is the first of six scheduled for the Democrats and will feature five candidates. Mrs. Clinton and Mr. Sanders are leading the field, but the showdown may be particularly crucial for former Maryland Gov.
Martin O’Malley,
who has been campaigning hard but still lags far behind in polls. Also on stage will be
Lincoln Chafee,
a former Rhode Island senator and governor, and former Virginia Sen.
Jim Webb,
who both have little to lose in attacking the front-runners.

Vice President
Joe Biden
was in Delaware over the weekend, where he was expected to meet with his family as he considers a presidential run. Either way, he doesn’t plan to participate in Tuesday’s debate.

Mr. O’Malley, for his part, is hoping the chance to sell his executive experience and liberal policy prescriptions to a prime-time audience will give his stalled candidacy some momentum.

“The only two candidates people have heard of are the inevitable front-runner and the senator from Vermont,” Mr. O’Malley said last week. “So once the debates happen, people will be able to hear from all of the candidates.”

Aides to Mr. Sanders say the senator needs to flesh out his big policies but also talk about incremental achievements he won during more than two decades in Congress.

“He understands he has to introduce himself to a big audience that doesn’t know about him,” said Tad Devine, Mr. Sanders’s senior strategist. “He needs to demonstrate a capacity to be president.”

To prepare, he has met with policy experts and read briefing materials, particularly on foreign affairs and military policy, but also the economy, and has run through possible questions with advisers.

“I am studying hard,” Mr. Sanders said in an interview last week. “There are hundreds of possible questions that can be asked, and we’re trying to figure out what they are, and how you can give a response to an important question in 30 seconds or 60 seconds.”

Mrs. Clinton also has been preparing by running mock debates with advisers led by
Ron Klain
and
Karen Dunn,
who played similar roles for President
Barack Obama
’s 2012 re-election. In the practice sessions, Mr. Sanders is played by Washington attorney
Robert Barnett
and Mr. O’Malley by Clinton aide
Jake Sullivan.

The former New York senator will seek to highlight her differences with Mr. Sanders without turning off a base of support that she would need next year if she wins the Democratic nomination. Until now, she has barely mentioned her closest rival.

Mr. Sanders has been subtly criticizing Mrs. Clinton for months as indecisive on such issues as the Asian trade agreement and the Keystone XL pipeline, both of which she ultimately opposed, and as unwilling to take on moneyed interests.

Sen. Bernie Sanders of Vermont waves to the crowd as he arrived on stage in Boulder, Colo., on Saturday.
Photo:
Cliff Grassmick/Associated Press

“People will have to contrast my consistency and my willingness to stand up to Wall Street and corporations, big corporations, with the secretary,” he said in an interview broadcast Sunday on NBC.

On many issues, the two agree about what direction the country needs to move, but he would take policy further to the left. He wants to raise the minimum wage higher than she does, for instance, and to put tougher rules on Wall Street than she would.

Some of Mrs. Clinton’s rationale is strategic. She often talks on the campaign trail about the value of bipartisanship and importance of notching smaller victories on the way to building bigger ones. Part of her pitch will be that she is better equipped than Mr. Sanders to accomplish the goals they share. Mr. Sanders suggests many of her ideas are too timid and urges his supporters to think big, not small, about changes needed.

Mrs. Clinton is also prepared to explain why her positions are the better choice, aides say. For instance, Mr. Sanders is proposing to make public colleges tuition-free, extending the nation’s commitment to free K-12 education through university. Mrs. Clinton wants families to contribute what they can, but would enable students to go to public colleges without taking on debt. Mrs. Clinton also might look to spotlight areas where her record or positions are more liberal, such as gun control and immigration.

“She needs to be aggressive in providing a contrast with Sen. Sanders without going over the top,” said
Jim Manley,
a Democratic strategist. “A lot of people will be tuning in to the campaign for the first time,‎and she is going to have to show them that she is ready to fight for their vote.”